Battery packs for portable products are typically available in a variety of capacities. U.S. Pat. No. 4,006,396 by Bogut, hereby incorporated by reference, discusses a universal battery charging scheme which provides an electrical element having a characteristic related to a predetermined charging rate of the battery in a voltage divider circuit. In this voltage divider measurement circuit, the measured voltage is directly related to the resistance of a coded resistor in accordance with the usual voltage divider relationship. This charging scheme is ideally utilized if the coded resistor's absolute value of 500 ohms, for example, would indicate a capacity of 500 mAH.
However, relying on the absolute value of the coded resistor for the correct operation of the above simple voltage divider measurement circuit is difficult to realize because of the various tolerances associated with the voltage divider circuit. Hence, a voltage tolerance window for each coded resistor instead of an absolute value needs to be delineated with each battery capacity value. Because of the potential overlapping of these voltage tolerance windows, the amount of available coded resistor values with their associated tolerance windows are limited. As new battery capacities become availabe, the need to distinguish among them correspondingly increases.
As presently implemented in a non-ideal manner in one application, the same resistor value indicates different capacities for different batteries of different portable devices such as different family types of two-way radios. This is the case, as seen in FIG. 2, where the coded resistors R18 of the A and C family type of radios are both 3,300 ohms, even though their capacities 2,240 mAH and 840 mAH, respectively. It is obvious that a battery charger generally cannot determine the correct charge capacity if two radios having different capacities were to have the same coded resistor.
Even if the same coded resistors of batteries having the same capacity are to be used with two different family types of radios, it is sometimes helpful to identify the radio family type. For example, one radio family type may have a different peak voltage level than another radio type that should not be exceeded when charging. This is the case, as seen in FIG. 2, where the coded resistors R18 of both the A and B family type of radios are both 2,400 ohms, since their capacities of 940 mAH are equal. However, the A type of radio can only be charged to 10.5 V while the B type of radio can exceed this level, as long as its maximum value is peaked at 11.2 V.
Thus, a need exists for a battery charging scheme that distinguishes between the variety of battery capacities, associated with different radio family types, available now and in the future for use in rechargeable batteries and battery packs, without changing the current non-ideal resistor coding scheme.